For users looking to take pictures quickly, when the phone is locked, simply press the home button twice -- then press the camera icon that appears to be taken directly to the camera. This works even when the phone is locked with a passcode.
Apple's engineers also added a feature that caused Camera+, a popular camera app, to be banned from the App Store for four months. Users of Apple's standard camera application can press the volume-up button to take a snapshot, rather than pressing the shutter button on the touchscreen. This makes the iPhone more like a standard camera, with the shutter release on the top.
Finally, as a corollary to the volume up shutter release, users can also plug in headphones with headphone controls -- like the headphones that are included with the iPhone -- and use the volume up on the headphones as a remote trigger for the camera.